S. R. Williams — Electromagnetic Effect. 297 



Art. XXX. — An Electromagnetic Effect; by S. R. 

 Williams. 



Under the above caption Bowden* in 1895 described some 

 interesting experiments on the behavior of a column of mercury 

 in a magnetic field when carrying an electrical current. I have 

 repeated his experiments with several variations, and inasmuch 

 as Bowden closes his short report by remarking that some of 

 the effects discovered by him are " difficult to understand," I 

 may be permitted to offer an explanation of the effects and at 

 the same time describe a very convenient method for measuring 

 magnetic field strengths. 



JBowderfs Experiments. 



Fig. 1 shows a sketch of the arrangement of Bowden's appa- 

 ratus. A horizontal glass tube connected two troughs, A and 

 B. Midway between A and B a vertical tube, V, was sealed 

 into the horizontal tube. This was then filled with mercury 

 as indicated and placed between the poles of an electromagnet. 

 The dotted line a, b, c, d, shows the position of the pole pieces. 

 The horizontal tube is normal to the magnetic field. If a 

 current flows along the horizontal tube of mercury from A to 

 B and the magnetic field is in toward the paper, then the mer- 

 cury will rise in the vertical tube, i. e., in the same direction 

 as the mechanical force acting upon a wire carrying a current 



in the same direction, AB, when placed in a similar magnetic 

 field. The glass tube is clamped in place and cannot move, 

 hence the mercury moves with respect to the tube. Why it 

 does is, perhaps, worthy of consideration. 



It has been observedf that, if a heavy electric current is 

 passed along a horizontal column of mercury, the column will 

 break in two, then reunite to repeat the process of breaking 

 again. This so-called "pinch-effect" has been explained from 

 the viewpoint that the column of mercury is composed of a 

 number of conducting filaments, each of which carrying a 

 current in the same direction would have a mutual attraction 

 for the other and, if this force is great enough, will crowd the 

 filaments together and pinch off the column of mercury. If 

 these filaments exist then, when placed in a magnetic field, as 

 indicated in fig. 1, they would be acted upon as a flexible wire 

 would be when carrying a current and placed in a magnetic 

 field. Hence if this flexible wire were placed in the horizontal 



* Bowden, Phil. Mag., vol. xl, p. 200, 1895. 

 + Nortkrup, Phys. Rev., vol. xxiv, p. 474, 1907. 



